Self-adaptive mechanisms are used in robotic fingers to provide the latter with the ability to adjust themselves to the shape of the object seized without any dedicated electronics, sensor or control. Because of the lack of commercial and industrial success of complex robotic hands, self-adaptive mechanisms have attracted much more interest from the research community and several prototypes have been built. Nevertheless, only a handful of prototypes are currently known.
Robotic hands are arguably the most popular end-effectors of robotic systems, at least within the research community. These devices have been developed with the aim of matching the human hands in terms of dexterity and adaptation capabilities, a difficult challenge with the currently available technologies. Robotic hands are often designed to equip either a dextrous manipulator for pick-and-place tasks or a human being as a prosthetic device. Several pioneer prototypes have been developed, some of them for more than two decades, and include the Stanford/JPL hand, as described by Salisbury, J. K., and Craig, J. J., in “Articulated Hands: Force Control and Kinematic Issues,” Int. J. Robot Res., 1(1), 1982, pp. 4-17, the Utah/MIT hand, as described by Jacobsen, S. C., Iversen, E. K., Knutti, D. F., Johnson, R. T., and Biggers, K. B., in “Design of the Utah/MIT Dextrous Hand,” proceedings of the IEEE International Conference on Robotics and Automation, San Francisco, Calif., Apr. 7-11, 1986, pp. 1520-1532, and the hands from the DLR, as described by Liu, H., Meusel, P., Seitz, N., Willberg, B., Hirzinger, G., and Jin, M. H., in “The Modular Multisensory DLR-HIT-Hand,” Mech. Mach. Theory, 42(5), 2007, pp. 612-625. However, these prototypes have all been designed aiming at an anthropomorphic architecture where each joint is independently actuated and the complexity of controlling often more than 12 axes, simultaneously and in real time, is especially demanding and yields significant costs. Hence, despite their capabilities, robotic hands have often little, if any, success outside research laboratories.
In the past few years, a significant increase in the development of innovative technologies for robotic hands has tried to address this issue. Significant efforts have been made to find designs simple enough to be easily built and controlled, and special emphasis has been placed on the reduction of the number of degrees of freedom (DOF), thereby decreasing the number of required actuators. A rapidly growing number of prototypes involve a smaller number of actuators without decreasing the number of DOF by taking advantage of self-adaptive mechanisms. These prototypes may be driven by tendons or linkages. This approach leads to the automatic and mechanical adaptation of the robotic finger to the shape of the object seized.
A well known example of a self-adaptive two-DOF finger driven by linkages and its closing sequence is illustrated in FIG. 5, and is similar to the configuration shown in U.S. Pat. No. 5,762,390, by Gosselin et al. The finger is actuated through the lower link (cf. arrow) and a spring with a mechanical limit is used to maintain the finger fully extended. The closing sequence occurs with a continuous motion of the actuator. Finally, both phalanges are in contact with the object and the finger has completed the shape adaptation. The actuator force is distributed between the two phalanges in contact with the object.
The properties required to achieve the self-adaptive behavior are usually not detailed in the literature, as they remain unclear. Only a few papers have been published dealing with theoretical aspects—i.e., valid not only for a single architecture but for a wide range of solutions—of self-adaptive mechanisms applied to grasping, namely, Shimojima, H., Yamamoto, K., and Kawakita, K., “A Study of Grippers with Multiple Degrees of Mobility,” JSME Int. J., 30(261), 1987, pp. 515-522, Laliberte, T., and Gosselin, C., “Simulation and Design of Underactuated Mechanical Hands,” Mech. Mach. Theory, 33(1), 1998, pp. 39-57, Hirose, S., “Connected Differential Mechanism and its Applications,” proceedings of the International Conference on Advanced Robotics, Tokyo, Japan, Sep. 9-10, 1985, pp. 319-325, and Birglen, L., Laliberté, T., and Gosselin, C., “Underactuated Robotic Hands,” Springer-Verlag, New York, 2008. Additionally, if the analysis of self-adaptive fingers is well detailed in the literature, as in Birglen et al., to the best of the author's knowledge, only the Shimojima et al. reference discusses the synthesis of such mechanisms. In the latter reference, only two-phalanx self-adaptive fingers are synthesized and six architectures are obtained.